The present invention relates to a rod lens array having a structure in which a large number of rod lens elements are combined into one unit while the large number of rod lens elements are arrayed in a plurality of rows, and an image-forming device using the rod lens array. More specifically, the invention relates to a rod lens array in which efficiency (brightness) can be improved in spite of the smallest number of lens rows by defining the relation between the number N of lens element rows and the degree m of overlapping of images while paying attention to the correlation therebetween, an image-forming device using the rod lens array, and a method of producing the rod lens array. This technique is useful for an image-forming device having a linear scanning type optical system such as a reading system of a scanner or a writing system of an LED printer.
A rod lens array is a light-convergent lens component having a structure in which a large number of rod lens elements each having a radially gradient index distribution are arrayed in parallel rows and, for example, impregnated/filled with a black silicone resin to thereby be combined into one unit. The rod lens array is designed so that images generated by adjacent lens elements are laid to overlap each other to thereby form one continuous correct unit-magnification image as a whole. Because the optical path length of the rod lens array is so short that it is unnecessary to use any inversion mirror, the rod lens array can be applied to a small-sized device. Hence, the rod lens array can be used popularly for an image-forming device in a linear scanning type optical system such as a scanner or a printer.
In the rod lens array used in such a linear scanning optical system, resolution is regarded as more important than brightness, so that the m value (the degree of overlapping of images) is selected to be a slightly large value. There is, however, the case where it is necessary to keep brightness as high as possible in accordance with the purpose. In this case, image unevenness is improved by electrical correction of light quantity and sensitivity. Even in this case, such a lens array having a good m value is used for the sake of suppression of cyclic light quantity unevenness.
As described above, when the rod lens array is used in a linear scanning type optical system, the brightness of the rod lens array decreases as the m value of the rod lens array increases under the condition that the number of lens rows is not changed. In the related art, the rod lens array used is not designed to be sufficiently advantageous in light quantity. Hence, in the related art, a large burden is imposed on a light source, or a sensor/photoconductor drum, so that the processing speed of the system (such as the reading speed of a scanner or the printing speed of a printer) is not improved as sufficiently as expected.
An object of the present invention is to provide a rod lens array having a structure advantageous in light quantity on the assumption of limitation in dimensions such as operating distance and conjugate length. Another object of the invention is to provide a multi-row rod lens array by which brightness and reduction of image unevenness can be achieved simultaneously to a certain degree, and a method of producing the rod lens array.
According to invention, there is provided a rod lens array having a structure in which a large number of rod lens elements are combined into one unit while the large number of rod lens elements are arrayed in a plurality of rows, wherein a number N of lens rows and a degree m of overlapping of images satisfy the relations:
{3(Nxe2x88x921)N/16}1/2 less than mxe2x89xa6{N(N+1)(28xe2x88x92N)/(9xe2x88x92N)}1/2/4,
and
Nxe2x89xa68
When m is equal to X0/D, in which D is a diameter of each lens, and X0 is a radius of a view field generated by each lens.
Further, according to the invention, there is provided a rod lens array having a structure in which a large number of rod lens elements are combined into one unit while the large number of rod lens elements are arrayed in a plurality of rows, wherein a number N of lens rows and a degree m of overlapping of images satisfy the relations:
{3(Nxe2x88x921)N/16}1/2 less than mxe2x89xa6{N(N+1)(58xe2x88x92N)/(19xe2x88x92N)}1/2/4.
Further, according to the invention, there is provided a rod lens array having a structure in which a large number of rod lens elements are combined into one unit while the large number of rod lens elements are arrayed in a plurality of rows, wherein a number N of lens rows and a degree m of overlapping of images satisfy the relations:
{3(Nxe2x88x921)N/16}1/2 less than mxe2x89xa6{3N(N+1)/16}1/2.
According to the invention, there is provided a rod lens array having a structure in which a large number of rod lens elements are combined into one unit while the large number of rod lens elements are arrayed in a plurality of rows, wherein a number N of lens rows and a degree m of overlapping of images satisfy the relations:
{3(Nxe2x88x921)(N+1)/16}1/2xe2x88x920.1xe2x89xa6mxe2x89xa6{3(Nxe2x88x921)(N+1)/16}1/2+0.1.
In these rod lenses, the number of lens rows is preferably selected to be not smaller than 6 in order to simultaneously achieve brightness and reduction of image unevenness to a certain degree.
Alternatively, according to the present invention, there is provided a rod lens array having a structure in which a large number of rod lens elements are combined into one unit while the large number of rod lens elements are arrayed in a plurality of rows, wherein a number N of lens rows, not smaller than 2 but not larger than 7, and a degree m of overlapping of images satisfy either one of the relations depending on a value of the number N:
in case of N=2, m is in a range of 0.61 to 1.32;
in case of N=3, m is in a range of 1.32 to 1.50;
in case of N=4, m is in a range of 1.50 to 2.18;
in case of N=5, m is in a range of 2.18 to 2.37;
in case of N=6, m is in a range of 2.37 to 3.04; and
in case of N=7, m is in a range of 3.04 to 3.24.
The present disclosure relates to the subject matter contained in Japanese patent application Nos. 2001-102951 (filed on Apr. 2, 2001) and 2002-39270 (filed on Feb. 15, 2002), where are expressly incorporated herein by reference in their entireties.